Calcium chloride hexahydrate is in many respects suited for use as a latent heat storage medium. The material in substantially pure form undergoes a liquid/solid phase transition at about 27.degree. C.-30.degree. C. (about 81.degree. F.-86.degree. F.) releasing or alternately absorbing about 46 calories of heat per gram (the latent heat of fusion). The material has particularly been considered for use as an encapsulated heat storage medium for use in solar energy space heating applications. Specific embodiments of such applications, for example, are taught in Report Nos. NSF/RANN/SE/C906/FR/76 1; NSF/RANN/SE/C906/TR/75 1; and ORO-5217-8, all available from the National Technical Information Service, Springfield, Virginia, and herein incorporated by reference. In order to improved the material for such applications, however, there exists a need to develop additives or a series of additives effective to modify the supercooling properties of hydrated calcium chloride compositions, and particularly additives which are highly effective in small amounts such as not to alter the advantageous heat storage properties of the material.